Risky Descent

June
28, 2007: NASA's Mars rover Opportunity is scheduled
to begin a descent down a rock-paved slope into the Red Planet's
massive Victoria Crater. This carries real risk for the long-lived
robotic explorer, but NASA and the Mars Rover science team
expect it to provide valuable science.

Opportunity
already has been exploring layered rocks in cliffs around
Victoria Crater. The team has planned the descent carefully
to enable an eventual exit, but Opportunity could become trapped
inside the crater or lose some capabilities. The rover has
operated more than 12 times longer than its originally intended
90 days.

Above:
Tracks left by Opportunity around the rim of Victoria
Crater. [More]

The
scientific allure is the chance to examine and investigate
the compositions and textures of exposed materials in the
crater's depths for clues about ancient, wet environments.
As the rover travels farther down the slope, it will be able
to examine increasingly older rocks in the exposed walls of
the crater.

"While
we take seriously the uncertainty about whether Opportunity
will climb back out, the potential value of investigations
that appear possible inside the crater convinced me to authorize
the team to move forward into Victoria Crater," said
Alan Stern, NASA associate administrator, Science Mission
Directorate, NASA Headquarters, Washington. "It is a
calculated risk worth taking, particularly because this mission
has far exceeded its original goals."

The
robotic geologist will enter Victoria Crater through an alcove
named Duck Bay. The eroding crater has a scalloped rim of
cliff-like promontories, or capes, alternating with more gently
sloped alcoves, or bays.

Above:
Cape St. Vincent, one of many promontories that jut out from
the walls of Victoria Crater. [More]

A
meteor impact millions of years ago excavated Victoria, which
lies approximately 4 miles south of where Opportunity landed
in January 2004. The impact-created bowl is half a mile across
and about five times as wide as Endurance Crater, where Opportunity
spent more than six months exploring in 2004.

The
rover began the journey to Victoria from Endurance 30 months
ago. It reached the rim at Duck Bay nine months ago. Opportunity
then drove approximately a quarter of the way clockwise around
the rim, examining rock layers visible in the promontories
and possible entry routes in the alcoves. Now, the rover has
returned to the most favorable entry point.

"Duck
Bay looks like the best candidate for entry," said John
Callas, rover project manager at NASA's Jet Propulsion Laboratory.
"It has slopes of 15 to 20 degrees and exposed bedrock
for safe driving."

If
all of its six wheels continue working, engineers expect Opportunity
to be able to climb back out of the crater. However, Opportunity's
twin rover Spirit lost the use of one wheel more than a year
ago, diminishing its climbing ability.

"These
rovers are well past their design lifetimes, and another wheel
could fail on either rover at any time," Callas said.
"If Opportunity were to lose the use of a wheel inside
Victoria Crater, it would make it very difficult, perhaps
impossible, to climb back out."

"We
don't want this to be a one-way trip," said Steve Squyres,
principal investigator for the rovers' science instruments,
Cornell University, Ithaca, N.Y. "We still have some
excellent science targets out on the plains that we would
like to visit after Victoria. But if Opportunity becomes trapped
there, it will be worth the knowledge gained."